A boiler furnace used at a thermal power plant needs to be opened during fabrication and periodically after starting operation so that a worker will enter the inside to conduct maintenance inspection. During maintenance inspection, it is necessary to define where an inspection point is, but it is difficult to accurately grasp the inspection point visually because the boiler capacity is large.
Therefore, the height position and lateral position of the inspection point have been conventionally measured using a measuring tape or the like to grasp where the worker is, i.e., a maintenance inspection position. However, not only does this method require a lot of time and manpower to grasp the position, but also some error could occur.
For this reason, use of a method, called a three-dimensional positioning system, to determine the position is conceivable. This method is to calculate distances from three or more positions whose position coordinates are known to a point whose position is to be determined using an acoustic wave from its propagation speed and propagation time in order to determine the position using the distances. Such a three-dimensional positioning system is disclosed in Patent Document 1 and Patent Document 2, for example. Another three-dimensional positioning system for determining the position using laser light instead of the acoustic wave is disclosed in Patent Document 3, for example.
A case where a maintenance inspection position in a boiler furnace is determined using a conventional three-dimensional positioning system will be described with reference to FIG. 14 and FIG. 15.
FIG. 14 is a perspective view illustrating a boiler furnace. Many pipes 102 are attached to a boiler furnace 101 near outer walls thereof as shown in FIG. 14, having a combustion chamber 103 inside thereof, with water wall tubes located along inner wall surfaces thereof.
When the operation of such a boiler furnace 101 is stopped to enable a worker to enter the inside in order to check for reduced thickness or corrosion conditions of the water wall tubes, inspection points need to be determined using the three-dimensional positioning system. Detailed description will be given with reference to FIG. 15.
FIG. 15 is a schematic diagram for explaining a conventional method of determining a position in the boiler furnace 101.
In FIG. 15, numeral 101 denotes the boiler furnace schematically shown. When the position of point A in the boiler furnace 101 is to be determined, wave receivers capable of receiving electric waves and acoustic waves are first placed at three reference positions R101, R102, and R103, whose position coordinates are known, in the boiler furnace 101, respectively. After that, an electric wave and an acoustic wave are transmitted from point A simultaneously, an arrival time interval between the electric wave and the acoustic wave is measured by the wave receivers placed at the three reference positions R101, R102, and R103, respectively, distances L101, L102, and L103 between position A and the respective reference positions R101, R102, and R103 are calculated using the arrival time interval and acoustic velocity, and the position of point A is determined using the distances L101, L102, and L103, and the position coordinates of the reference positions R101, R102, and R103.
However, distances L101′, L102′, and L103′ from point A′, which is symmetrical to point A with respect to a plane formed by the three reference positions R101, R102, and R103, to the three reference positions R101, R102, and R103 have the following relations: L101=L101′, L102=L102′, and L103=L103′.
In other words, points to which distances from the three reference positions R101, R102, and R103 are L101, L102, and L103 exist at two positions (A and A′). If the position corresponding to point A′ shown in FIG. 15 is located outside the boiler furnace 101, since the position corresponding to point A′ can be excluded from candidates for point A, point A can be determined. However, if point A′ shown in FIG. 15 is located inside the boiler furnace 101, the position of point A cannot be determined.
In Patent Document 1, since position determination is made on condition that it is known in which direction the point to be determined is positioned with respect to the plane formed by the three reference positions, the point positioned symmetrically to the point to be determined with respect to the plane formed by the three reference positions is not considered. In Patent Document 2, since the three reference positions are located on the ground to determine a position in the air, the point positioned symmetrically to the point to be determined with respect to the plane formed by the three reference positions is located in the ground and hence is not considered as well. Thus, a position in the boiler furnace cannot be always determined even if either of the methods disclosed in Patent Documents 1 and 2 is employed.
Further, the position calculation method disclosed in Patent Document 2 is to obtain distances between a fix and known positions from the arrival times of an acoustic wave, so that the time when the acoustic wave is transmitted from the fix needs counting. To this end, a wave transmitter needs electric wave transmitting means separately to transmit wave transmitting time to an arithmetic unit, resulting in enlargement of equipment. In addition, if a scaffold is set up near a boiler sidewall on which a position is to be determined during inspection or the like in the boiler furnace, the scaffold becomes an obstacle to transmission and reception of the acoustic wave, and it is expected that use of the method disclosed in Patent Document 2 will reduce positioning accuracy.
Further, in the method using laser light as disclosed in Patent Document 3, if there is an obstacle to interrupt laser light between a transmitter and a receiver, measurement is impossible and hence the method is not suited to use for inspection in a boiler furnace where there are many obstacles such as the scaffold at the time of inspection. In addition, since it is dangerous if laser light gets in eyes, laser light intensity is limited.
Further, in order to enable determination of direction to a point (inspection position) to be determined from a plane formed by three points even in the boiler furnace, it is considered, for example, that all the three reference points are placed on the floor surface at the bottom of the boiler furnace. However, when the three reference points are placed on the floor surface, there is the scaffold that becomes an obstacle to acoustic wave propagation between the wave transmitting position and the wave receiving position, causing reduction in positioning accuracy.
In addition, since the capacity of the boiler furnace is large, plural workers enter the inside during major inspection or overhaul. Therefore, although it is necessary to detect inspection positions of the plural workers, any of Patent Documents 1, 2, and 3 is not available for position detection of plural workers.
[Patent Document 1] Japanese Patent Application Laid-Open No. 63-266376
[Patent Document 2] Japanese Patent Application Laid-Open No. 2004-108978
[Patent Document 3] Japanese Patent Application Laid-Open No. 3-251706